1. Technical Field
The present disclosure generally relates to battery power sources, in particular to an active battery balancing circuit, to a method of balancing an electric charge in a plurality of cells of a battery, and to a power source wherein an active battery balancing circuit is employed.
2. Description of the Related Art
Rechargeable batteries are used as a power source for various devices, from small devices having a relatively low power consumption such as mobile phones, digital cameras and media players to devices that have a high power consumption and/or require a voltage of up to several hundred Volt such as electric vehicles and hybrid vehicles, in particular electric and hybrid cars. Besides lead acid batteries, nickel cadmium batteries, and nickel metal hydride batteries, lithium ion batteries have recently gained popularity, due to their high energy-to-weight ratio, absence of memory effect and slow loss of charge when not in use.
In applications wherein a voltage that is greater than the voltage of a single cell (approximately 2 Volt for a lead acid cell, 1.2 Volt for nickel cadmium and nickel metal hydride cells, and approximately 3.6 Volt for lithium ion cells), a plurality of cells are electrically connected in series. For achieving higher operating voltages of about 200 to about 300 or more Volts, as may be used in electric or hybrid car applications, stacks comprising many cells which are electrically connected in series are used.
Battery cells can be damaged when they are used outside an allowable voltage range, both when they are charged to a voltage that is greater than a maximum allowed voltage (overcharge), and when they are deeply discharged such that the voltage between the poles of the cell is smaller than a particular minimum voltage that depends on the type of the cell. Lithium ion cells can be particularly susceptible to damage outside of an allowable voltage range.
In a battery comprising a plurality of cells, the individual cells can have a different capacity, for example due to production tolerances, uneven temperature distribution in the battery, and differences in the aging characteristics of the cells. Moreover, the individual cells in a battery can have a different state of charge, due to fluctuations that may occur in the charge or discharge process of the battery.
If there is a degraded cell having a diminished capacity in the battery, or a cell having a higher charge than the other cells, there is a danger that once such a cell has reached its full charge, it will be subject to overcharging until the other cells of the battery reach their full charge. The result can be temperature and pressure build up and possible damage to the cell. During discharge, a cell having a smaller capacity than the other cells, or a lower charge, will have the greatest depth of discharge and will tend to fail before the other cells. It is even possible for the voltage on the cell(s) having the lowest state of charge and/or the smallest capacity to be reversed as the cell(s) become fully discharged before the rest of the cells, which may lead to a premature failure of the cell.
For overcoming these problems, it has been proposed to balance the charge of the individual cells, using specific charge balancing circuits connected to the cells of a battery and/or incorporated into the battery. Such charge balancing circuits can be classified as passive and active balancing circuits.
A passive battery balancing circuit according to the state of the art will be described with reference to FIG. 1.
In FIG. 1, reference numeral 111 denotes a battery comprising a plurality of cells 101, 102 that are electrically connected in series between a positive pole 112 and a negative pole 113 of the battery 111. In addition to cells 101, 102, further cells can be present in the battery 111, as schematically indicated by three dots between cell 101 and the positive pole 112.
Each of the cells 101, 102 has a positive pole and a negative pole. In accordance with conventional circuit symbols, cells 101, 102 are shown as a pair of parallel lines, wherein the longer line denotes the positive pole, and the shorter line denotes the negative pole.
A passive balancing circuit 100 is connected to the battery 111. The balancing circuit 100 comprises a plurality of bypass resistors 103, 104 and switches 105, 106. The switches 105, 106 can be solid state switches, each comprising one or more transistors, and can be operated by a control circuit (not shown) of the balancing circuit 100.
By closing a respective one of the switches 105, 106, each of the bypass resistors 103, 104 can be connected between the poles of one of the cells 101, 102. For example, by closing the switch 105, the bypass resistor 103 can be connected between the poles of the cell 101. If this is done while the battery 111 is charged, a part of the current applied to the battery 111 flows through the bypass resistor 103 instead of flowing into the cell 101, such that the cell 101 is charged to a smaller extent than it would be if the switch 105 were open. If the switch 105 is closed while the battery 111 is discharged, or while the battery 111 is idle, a current can flow between the positive and the negative pole of the cell 101 through the bypass resistor 103, such that the cell 101 is discharged. In either case, by closing the switch 105, the amount of charge in cell 101 is reduced compared to a case wherein the switch 105 is open. Thus, the charge of the cells 101, 102 can be balanced by closing switches associated with cells having a particularly high state of charge, for selectively reducing the amount of charge stored in these cells relative to the charge of other cells of the battery 111.
A disadvantage of this approach is that passive charge balancing leads to a loss of energy in the battery 111, since energy is converted to heat in the bypass resistors 103, 104.
For avoiding such a loss of energy, active balancing has been proposed, wherein charge and energy are transferred between the cells 101, 102 of a battery 111.
An active battery balancing circuit 200 according to the state of the art will be described with reference to FIG. 2. The balancing circuit 200 comprises a capacitor 201 and switches 202, 203. Similar to the switches 105, 106 described above with reference to FIG. 1, the switches 202 can be solid state switches, and can be operated by a control unit (not shown).
One of the terminals of the capacitor 201 can alternatively be connected to the positive pole and the negative pole of the cell 101 by operation of the switch 202, and the other terminal of the capacitor 201 can alternatively be connected to the positive and negative pole of the cell 102 by operating the switch 203. Similar arrangements of capacitors and switches can be provided between other pairs of cells in the battery 111. In the balancing circuit 200, energy and charge can be transferred from a cell of the battery 111 to capacitor 201 and from capacitor 2001 to another cell having a lower voltage.
As an example, a case will be described wherein the cell 101 has a higher voltage than the cell 102. First, the switches 202, 203 are operated such that the capacitor 201 is connected between the positive and the negative pole of the cell 101. In doing so, the capacitor 201 is charged to approximately the voltage between the poles of the cell 101. Thereafter, the switches 202, 203 are operated such that the capacitor 201 is connected between the poles of the cell 102. Since the voltage of the cell 102 is smaller than the voltage of the cell 101, and the capacitor 201 has been charged to the voltage of the cell 101, the capacitor 201 now has a higher voltage than the cell 102, such that charge from the capacitor 201 is flowing into the cell 102 until the voltage of the capacitor 201 equals the voltage of the cell 102. Thereby, the cell 101 is partially discharged, and cell 102 is charged. Subsequently, the process can be repeated.
A disadvantage of the battery balancing circuit 200 is that the operation of the balancing circuit 200 can be relatively inefficient, such that a relatively large fraction of the energy transferred from cells 101, 102 to capacitor 201 is lost.
FIG. 3 shows another active battery balancing circuit 300 according to the state of the art. The balancing circuit 300 comprises an inductance 301, for example a coil, and a switch 302 that can, for example, be a solid state switch operated by a control unit (not shown). By operating the switch 302, the inductance 301 can alternatively be connected between the poles of the cell 101 and the poles of the cell 102. Similar circuitry comprising an inductance and a switch can be provided between other pairs of cells in the battery 111.
For transferring energy and charge from one of the cells 101, 102 of the battery 111, for example cell 101, to another one of the cells of the battery 111, for example cell 102, the switch 302 can be operated for connecting the coil 301 between the poles of the cell 101. Thus, an electric current through the inductance 301 begins to flow between the poles of the cell 101. The current increases with time, and an increasing magnetic field is built up in the inductance 301. Thereafter, the switch 302 is operated for connecting the inductance 301 between the poles of the cell 102. Since energy is stored in the magnetic field in the inductance 301, an electric current continues to flow through the inductance 301. Since the inductance 301 is now connected to the cell 102, this current is flowing into the cell 102. Hence, the battery cell 101 with the higher voltage is discharged with a current and the cell 102 with the lower voltage is charged with that current.
A disadvantage of the balancing circuit 300 is that it allows a transfer of energy and charge only between pairs of cell connected to the same inductance, which are usually neighboring cells of the battery 111. Thus, a series of charge transfer processes between neighboring cells of the battery 111 can be necessary for shifting energy and charge from the cell having the highest voltage to the cell having the lowest voltage. Since losses may occur in each of the charge transfer processes, this may lead to a relatively high loss of energy.
FIG. 4 shows yet another active battery balancing circuit 400 according to the state of the art.
The balancing circuit 400 comprises a transformer 401 having a primary winding 405 and a plurality of secondary windings 402, 403 that are wound around a common core. The primary winding 405 can be connected between the positive pole 112 and the negative pole 113 of the battery 111 by means of switch 408. The secondary windings 402, 403 can each be connected between the positive and negative pole of one of the cells 101, 102, and disconnected therefrom by means of a respective switch 406,407. The switches 406, 407, 408 can be solid state switches, and can be controlled by a control unit (not shown).
The balancing circuit 400 can be used for selectively charging one of the cells 101, 102, for example, the cell having the lowest voltage. For this purpose, the switch 408 is closed while the switches 406, 407 are open. Thus, an electric current flows from entire battery 111 through the primary winding 405 of the transformer 401, and a magnetic field is built up in the core of the transformer 401. Thereafter, the switch 408 is opened, and a switch associated with the cell to be charged, for example, the switch 406 associated with cell 101, is closed. The energy stored in the magnetic field in the core of the transformer 401 creates a current in the secondary winding 402 that flows into the cell 101. Thus, the cell 101 is charged, and the charge in the other cells of the battery 111 is reduced.
Alternatively, the balancing circuit 400 can be used for selectively discharging one of the cells 101, 102 of the battery 111, for example the cell having the highest voltage, and charging the entire battery 111 with charge drawn from this cell. For example, for selectively discharging the cell 102, the switch 407 can be closed, with the switches 406, 408 being open. A current flows from the cell 102 through the secondary winding 403, and creates a magnetic field in the core of the transformer 401. Then, the switch 407 is opened, and the switch 408 is closed. The presence of the magnetic field in the core of the transformer 401 creates an electric current in the primary winding 405 of the transformer 401, and the energy stored in the magnetic field flows into the entire battery 111.
Thus, the balancing circuit 400 allows to selectively discharge cells of the battery 111 having a high voltage, for example during the charging of the battery 111, and to selectively charge cells of the battery 111 having a low voltage, for example during the discharge of the battery 111.
A problem of the balancing circuit 400 is, that the transformer 401 can be a relatively bulky component, in particular if the battery 111 comprises a large number of cells and, hence, the transformer comprises a large number of secondary coils.
Moreover, the voltage that is applied to the entire battery 111 when one of the cells is selectively discharged, and the voltage that is applied to a cell of the battery 111 when one of the cells is selectively charged, depend on the inductive coupling between the secondary windings 402, 403 and the primary winding 405. If there are differences in the inductive coupling of the individual secondary windings 402, 403 and the primary winding 405, voltage differences may occur in the selective discharging and charging of the cells 101, 102 of the battery 111. Thus, the balancing circuit 400 can contribute to the creation of different voltages of the cells 101, 102, rather than leading to an averaging of the voltages of the cells 101, 102. Therefore, the balancing circuit 400 may require a precise balancing of the inductive coupling between the primary winding 405 and the secondary windings 402, 403, which may increase the costs of the balancing circuit 400, in particular in case of a large number of secondary windings.